Semiconductor device and manufacturing method thereof

ABSTRACT

A conventional DRAM needs to be refreshed at an interval of several tens of milliseconds to hold data, which results in large power consumption. In addition, a transistor therein is frequently turned on and off; thus, deterioration of the transistor is also a problem. These problems become significant as the memory capacity increases and transistor miniaturization advances. A transistor is provided which includes an oxide semiconductor and has a trench structure including a trench for a gate electrode and a trench for element isolation. Even when the distance between a source electrode and a drain electrode is decreased, the occurrence of a short-channel effect can be suppressed by setting the depth of the trench for the gate electrode as appropriate.

TECHNICAL FIELD

The present invention relates to a technique for miniaturizing semiconductor integrated circuits. The invention disclosed in this specification includes in its scope an element formed using a compound semiconductor, in addition to that formed using a silicon semiconductor, as a component of a semiconductor integrated circuit, and discloses an element formed using an oxide semiconductor as an example.

BACKGROUND ART

As semiconductor memory devices, dynamic RAMs (DRAMs) are well-known products and currently used in a variety of electronic devices. A memory cell which is a key component in a DRAM includes a read and write transistor and a capacitor.

Circuit patterns for DRAMs, like those for other semiconductor integrated circuits, have been miniaturized in accordance with the scaling law, and there was a time when it was considered difficult to achieve a design rule of 100 nm or less. One of the reasons is that in a transistor having a channel length of 100 nm or less, a punch-through current is likely to flow due to a short-channel effect and the transistor becomes incapable of functioning as a switching element, which has been considered to be a problem. In order to prevent a punch-through current, a silicon substrate may be doped with an impurity at high concentration. However, this is not an appropriate solution to the problem because it makes a junction leakage current likely to flow between a source and the substrate or between a drain and the substrate and eventually causes a deterioration of memory retention characteristics.

Against such a problem, a method has been considered for reducing the area occupied by one memory cell and also maintaining an effective channel length so as not to cause a short-channel effect by forming a three-dimensional transistor in the memory cell. One example is a structure in which a U-shaped vertically long groove is formed in a region where a channel portion of a transistor is formed, a gate insulating film is formed along a wall surface in the groove, and a gate electrode is formed so as to fill the groove (see Reference 1).

A transistor having a channel portion of such a structure has a long effective channel length because a current flows between a source region and a drain region via an indirect route across the groove portion. This provides an advantageous effect of reducing the area occupied by a transistor in a memory cell and suppressing a short-channel effect.

REFERENCE

-   [Reference 1] Kinam Kim, “Technology for sub-50 nm DRAM and NAND     Flash Manufacturing”, International Electron Devices Meeting 2005,     IEDM Technical Digest, December 2005, pp. 333-336

DISCLOSURE OF INVENTION

However, a conventional DRAM needs to be refreshed at an interval of several tens of milliseconds to hold data, which results in large power consumption. In addition, a transistor therein is frequently turned on and off; thus, deterioration of the transistor is also a problem. These problems become significant as the memory capacity increases and transistor miniaturization advances.

Thus, it is an object of the present invention to provide a technique that can improve data retention characteristics of a semiconductor memory device. Another object is to provide a technique that can reduce power consumption as well as improving data retention characteristics of a semiconductor memory device.

In order to achieve any of the above objects, a circuit, specifically a semiconductor memory device, is formed using a transistor including an oxide semiconductor (OS), particularly a MOS transistor including an oxide semiconductor. The oxide semiconductor is a substantially intrinsic semiconductor. For that reason, there is the advantage that the off-state current is extremely small.

With the use of the transistor including an oxide semiconductor, the interval between refresh operations can be longer than that for a conventional DRAM, and power consumption can be reduced. In addition, the number of times a transistor is turned on and off per unit time can be reduced, and therefore, the lifetime of the transistor can be made longer than that in a conventional DRAM.

Even in a transistor including an oxide semiconductor layer, a short-channel effect might be caused with the advancement of transistor miniaturization. In view of this, a novel transistor structure including an oxide semiconductor layer is proposed.

One embodiment of the present invention is a semiconductor device which includes a first trench and a second trench in an insulating layer, an oxide semiconductor layer in contact with a bottom surface and an inner wall surface of the first trench, a gate insulating layer over the oxide semiconductor layer, a gate electrode over the gate insulating layer, and an insulating layer filling the second trench, in which the gate insulating layer is over a bottom surface and an inner wall surface of the second trench, and the gate electrode fills the first trench. The first trench is a trench for the gate electrode, and the second trench is a trench for element isolation. Note that an upper surface shape of the first trench is a stripe shape or a rod-like shape, and an upper surface shape of the second trench is a lattice shape, a stripe shape, or a rod-like shape.

In the above structure, the semiconductor device may further include a source electrode or a drain electrode in contact with the oxide semiconductor layer.

In the above structure, the gate insulating layer may be in contact with and cover a side surface of the oxide semiconductor layer in order to improve reliability of a transistor.

The thickness of the oxide semiconductor layer is preferably greater than or equal to 1 nm and less than or equal to 100 nm, and the oxide semiconductor layer may be a crystalline oxide semiconductor layer. With the crystalline oxide semiconductor layer, it is possible to provide a highly reliable semiconductor device in which changes of the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light can be suppressed. Furthermore, the crystalline oxide semiconductor layer has neither a single crystal structure nor an amorphous structure and is a crystalline oxide semiconductor film having c-axis alignment (also referred to as c-axis aligned crystalline oxide semiconductor (CAAC-OS) film). The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases. Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation.

With use of the CAAC-OS film in a transistor, change in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability.

The cross-sectional shape of the oxide semiconductor layer in the channel-length direction is a shape curved along the cross-sectional shape of the first trench, that is, a U shape. With this structure, as the first trench becomes deeper, the channel length of a transistor increases. In the case where the oxide semiconductor layer is a crystalline oxide semiconductor layer, the oxide semiconductor layer contains a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer having a U shape.

In the transistor having a trench structure disclosed in this specification, the occurrence of a short-channel effect can be suppressed by appropriately setting the depth of the first trench even when the distance between the source electrode and the drain electrode is decreased.

Improvement of data retention characteristics of a semiconductor memory device can be achieved. A reduction of power consumption as well as improvement of data retention characteristics of a semiconductor memory device can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are cross-sectional views and a top view of one embodiment of the present invention.

FIGS. 2A to 2C are cross-sectional schematic views of one embodiment of the present invention.

FIGS. 3A and 3B are a cross-sectional view and a circuit diagram of one embodiment of the present invention.

FIG. 4 is a cross-sectional view of one embodiment of the present invention.

FIGS. 5A and 5B are a circuit diagram and a conceptual diagram of a semiconductor device of one embodiment of the present invention.

FIG. 6 is a cross-sectional view of one embodiment of the present invention.

FIGS. 7A and 7B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 8A and 8B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 9A and 9B are a cross-sectional view of a structure used for calculation and results of the calculation.

FIGS. 10A and 10B are circuit diagrams of one embodiment of the present invention.

FIG. 11 is a block diagram of a portable device of one embodiment the present invention.

FIG. 12 is a block diagram of a semiconductor device of one embodiment of the present invention.

FIG. 13 is a block diagram of an electronic book of one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail below with reference to drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details of the present invention can be modified in various ways. In addition, the present invention should not be construed as being limited to the description in the embodiments given below.

Embodiment 1

In this embodiment, a structure of a transistor and a method for manufacturing the transistor according to one embodiment of the present invention will be described with reference to FIGS. 1A to 1C and FIGS. 2A to 2C. FIG. 1A is an example of a cross-sectional view of a transistor 162 in a channel-length direction. FIG. 1B is an example of a cross-sectional view perpendicular to an element isolation region 165 between the transistor 162 and a transistor 163. FIG. 1C is an example of a top view of the transistor 162 and the transistor 163. Note that FIG. 1B is part of a cross-sectional view of the transistor 162 in a channel-width direction, and corresponds to a cross-sectional view taken along a dotted line D1-D2 in FIG. 1C. FIG. 1A corresponds to a cross-sectional view taken along a dotted line A1-A2 in FIG. 1C.

First, an insulating layer 130 is formed with an oxide film over a semiconductor substrate. Then, a plurality of trenches (also referred to as grooves) is formed in the insulating layer 130. Then, an oxide semiconductor layer 144 is formed so as to cover the trenches. The trenches can be formed using a known technique; in this embodiment, trenches having a depth of approximately 0.4 μm are formed. In addition, the trenches for gate electrodes are formed in a single etching step or through a plurality of etching steps.

As the semiconductor substrate, an SOI substrate, a semiconductor substrate provided with a driver circuit including a transistor with a MOSFET structure, a semiconductor substrate provided with a capacitor, or the like is used.

The insulating layer 130 is in contact with the oxide semiconductor layer 144 and therefore preferably contains a large amount of oxygen which exceeds at least the stoichiometry in (a bulk of) the layer. For example, in the case where a silicon oxide film is used as the insulating layer 130, the composition formula is SiO_(2+α) (α>0). By using the insulating layer 130 described above, oxygen can be supplied to the oxide semiconductor layer 144 and favorable characteristics can be obtained.

The oxide semiconductor layer 144 can have a thickness of 1 nm to 100 nm and can be formed by a sputtering method, a molecular beam epitaxy (MBE) method, a CVD method, a pulse laser deposition method, an atomic layer deposition (ALD) method, a coating method, a printing method, or the like as appropriate. The oxide semiconductor layer 144 may be formed using a sputtering apparatus which performs film formation with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target, which is so called a columnar plasma (CP) sputtering system.

As a material of the oxide semiconductor layer 144, at least one element selected from In, Ga, Sn, and Zn is contained. For example, a four-component metal oxide such as an In—Sn—Ga—Zn—O-based oxide semiconductor, a three-component metal oxide such as an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or a Sn—Al—Zn—O-based oxide semiconductor, a two-component metal oxide such as an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxide semiconductor, a single-component metal oxide such as an In—O-based oxide semiconductor, a Sn—O-based oxide semiconductor, or a Zn—O-based oxide semiconductor, or the like can be used. In addition, any of the above oxide semiconductors may contain an element other than In, Ga, Sn, and Zn, for example, SiO₂. Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or more lanthanoids selected from lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) may be contained.

For example, an In—Ga—Zn—O-based oxide semiconductor means an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the ratio of In:Ga:Zn. Further, a metal element in addition to In, Ga, and Zn may be contained.

For the oxide semiconductor layer, a thin film represented by the chemical formula, InMO₃(ZnO), (m>0) can be used. Here, M represents one or more metal elements selected from Zn, Ga, Al, Mn, and Co. For example, Mmay be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably, In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), further preferably, In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

As a material of the oxide semiconductor layer 144, a crystalline oxide semiconductor containing In and having c-axis alignment is preferably used. There are three methods for obtaining a crystalline oxide semiconductor having c-axis alignment. The first method is to form the oxide semiconductor layer 144 at a film formation temperature of 400° C. to 450° C., thereby obtaining c-axis alignment in the directions of arrows in FIG. 2A. The second method is to form a thin oxide semiconductor film and then subject the film to heat treatment performed at 200° C. to 700° C., thereby obtaining c-axis alignment in the directions of arrows in FIG. 2B. The third method is to form a first thin oxide semiconductor film, subject the film to heat treatment performed at 200° C. to 700° C., and then form a second oxide semiconductor film, thereby obtaining c-axis alignment in the directions of arrows in FIG. 2C.

As illustrated in FIGS. 2A to 2C, through any of the methods, crystal growth occurs in the direction perpendicular to an uneven surface of the oxide semiconductor layer 144, and a crystalline oxide semiconductor having c-axis alignment can be obtained.

Next, electrodes 142 a and 142 b each of which functions as a source electrode or a drain electrode are formed in contact with the oxide semiconductor layer 144. The electrodes 142 a and 142 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component.

In addition, insulating layers 143 a and 143 b are formed in order to protect the electrodes 142 a and 142 b. Next, planarization treatment is performed using chemical mechanical polishing (CMP) or the like. In this planarization treatment, the insulating layers 143 a and 143 b function as buffer layers for preventing the electrodes 142 a and 142 b from being removed.

Next, trenches for element isolation in the channel-length direction and trenches for element isolation in the channel-width direction are formed. These trenches for element isolation may have a continuous upper surface pattern shape or separate upper surface pattern shapes. In this embodiment, division of the oxide semiconductor layer is achieved by formation of the trenches; thus, these trenches have a continuous upper surface pattern shape (a lattice shape) in FIG. 1C. During the formation of the trenches for element isolation in the channel-width direction, division into the electrode 142 a and the electrode 142 b can also be achieved. Note that the timing of formation of the trenches for element isolation is not particularly limited. In addition, the depth of the trenches for element isolation is not limited to a depth at which the horizontal position of the bottoms thereof is the same as that of the bottoms of the trenches for the gate electrodes, as long as sufficient element isolation can be achieved. Element isolation can be ensured by setting the horizontal position of the bottoms of the trenches for element isolation to be deeper than that of the bottoms of the trenches for the gate electrodes.

Then, a gate insulating layer 146 is formed so as to cover part of the oxide semiconductor layer 144, the electrodes 142 a and 142 b each functioning as a source electrode or a drain electrode, and the insulating layers 143 a and 143 b. The gate insulating layer 146 is also formed on the inner walls and bottoms of the trenches for element isolation in the channel-length direction and the inner walls and bottoms of the trenches for element isolation in the channel-width direction.

The gate insulating layer 146 can have a thickness of 1 nm to 100 nm and can be formed by a sputtering method, an MBE method, a CVD method, a pulse laser deposition method, an ALD method, a coating method, a printing method, or the like as appropriate. The gate insulating layer 146 may be formed using a sputtering apparatus which performs film formation with surfaces of a plurality of substrates set substantially perpendicular to a surface of a sputtering target, which is so called a columnar plasma (CP) sputtering system.

The gate insulating layer 146 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. A portion of the gate insulating layer 146 which is in contact with the oxide semiconductor layer 144 preferably contains oxygen. In particular, an insulating film in contact with the oxide semiconductor layer 144 preferably contains a large amount of oxygen which exceeds at least the stoichiometry in (a bulk of) the layer. For example, in the case where a silicon oxide film is used as the gate insulating layer 146, the composition formula is SiO_(2+α) (α>0). In this embodiment, a silicon oxide film of SiO_(2+α) (α>0) is used as the gate insulating layer 146. By using the silicon oxide film as the gate insulating layer 146, oxygen can be supplied to the oxide semiconductor layer 144 and favorable characteristics can be obtained. Further, the gate insulating layer 146 is preferably formed in consideration of the size of a transistor to be formed and the step coverage with the gate insulating layer 146.

When the gate insulating layer 146 is formed using a high-k material such as hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), gate leakage current can be reduced. Further, the gate insulating layer 146 may have a single-layer structure or a stacked structure.

Then, a gate electrode 148 a is formed over the gate insulating layer 146 so as to fill the trench for the gate electrode. The gate electrode 148 a can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium, neodymium, or scandium or an alloy material which contains any of these materials as its main component. The gate electrode 148 a may have a single-layer structure or a stacked structure.

As one layer of the gate electrode 148 a which is in contact with the gate insulating layer 146, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (InN, SnN, or the like) film is used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage of the transistor to be positive when used as the gate electrode. Accordingly, a so-called normally off switching element can be provided.

When the gate electrode 148 a is formed in the trench for the gate electrode, the transistor 162 with a trench structure is formed.

Then, an insulating layer 149 is formed so as to cover the gate electrode 148 a and a gate electrode 148 b. As the insulating layer 149, an insulating film providing favorable step coverage is preferably used. The insulating layer 149 can be formed using a silicon oxide film, a gallium oxide film, an aluminum oxide film, a silicon nitride film, a silicon oxynitride film, an aluminum oxynitride film, or a silicon nitride oxide film. In this embodiment, an aluminum oxide film is used as the insulating layer 149. In FIGS. 1A and 1B, the gate insulating layer 146 is formed in contact with a side surface of the oxide semiconductor layer 144, and furthermore, the insulating layer 149 is formed. Accordingly, in this embodiment, a silicon oxide film of SiO_(2+α) (α>0) covers a side surface of the oxide semiconductor layer 144 and an aluminum oxide film covers the silicon oxide film, thereby blocking oxygen so as not to be diffused from the silicon oxide film and pass through the insulating layer 149.

After the insulating layer 149 is formed, an insulating layer 150 for filling the trenches for element isolation is formed by a CVD method or the like. By filling the trenches for element isolation with the insulating layer 150, element isolation regions 161 and 165 are formed. Note that when the gate insulating layer 146 and the insulating layer 149 are stacked in the trenches for element isolation before the insulating layer 150 is formed, regions to be filled with the insulating layer 150 can be smaller and can be smoothly filled with the insulating layer 150. After that, planarization treatment is performed using CMP or the like, whereby the structure illustrated in FIGS. 1A and 1B can be obtained.

As illustrated in FIG. 1B, a space between the gate electrode 148 a of the transistor 162 and the gate electrode 148 b of the transistor 163 adjacent thereto is also filled with the insulating layer 150, which makes it possible to prevent a short-circuit between the gate electrodes. Furthermore, as illustrated in FIG. 1A, a space between the electrode which functions as a source electrode or a drain electrode of the transistor 162 and an electrode which functions as a source electrode or a drain electrode of a transistor adjacent thereto in the channel-length direction is also filled with the insulating layer 150, which makes it possible to prevent a short-circuit between these electrodes.

A channel of a transistor is formed along an inner wall of a trench, and carriers smoothly flow through In—O—In—O in a crystalline oxide semiconductor (CAAC) even when its channel formation region is not flat. In this embodiment, the oxide semiconductor layer 144 is formed in contact with the inner wall of the trench of 0.4 μm; thus, the channel length is approximately 0.8 μm or more. A transistor with a channel length of 0.8 μm or more can be a normally off transistor, and the occurrence of short-channel effect can be prevented. In addition, by employing the trench structure, a reduction in the planar area of a transistor can be achieved, so that higher integration can be achieved.

Embodiment 2

FIGS. 3A and 3B illustrates an example of a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, and which has an unlimited number of write cycles.

Since the off-state current of the transistor 162 is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption.

FIG. 3A illustrates an example of a cross section of the semiconductor device.

The semiconductor device illustrated in FIG. 3A includes a transistor 160 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. The transistor 162 is the same as that in Embodiment 1; thus, for description of FIGS. 3A and 3B, the same reference numerals are used for the same parts as those in FIG. 1A.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics.

Although both of the above transistors are n-channel transistors in the following description, it is needless to say that p-channel transistors can be used. The technical nature of the disclosed invention is to use an oxide semiconductor in the transistor 162 so that data can be held. Therefore, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to the structure described here.

The transistor 160 in FIG. 3A includes a channel formation region 116 provided in a substrate 100 including a semiconductor material (such as silicon), impurity regions 120 provided such that the channel formation region 116 is sandwiched therebetween, metal compound regions 124 provided in contact with the impurity regions 120, a gate insulating layer 108 provided over the channel formation region 116, and a gate electrode 110 provided over the gate insulating layer 108.

An electrode 126 is connected to part of the metal compound region 124 of the transistor 160. Here, the electrode 126 functions as a source electrode or a drain electrode of the transistor 160. Further, an element isolation insulating layer is formed on the substrate 100 so as to surround the transistor 160, and an insulating layer 130 is formed so as to cover the transistor 160. Note that for higher integration, it is preferable that, as in FIG. 3A, the transistor 160 does not have a sidewall insulating layer. On the other hand, when the characteristics of the transistor 160 have priority, the sidewall insulating layer may be formed on a side surface of the gate electrode 110 and the impurity regions 120 may include a region having a different impurity concentration.

As illustrated in FIG. 3A, the transistor 162 includes the oxide semiconductor layer 144 and has a trench structure.

Here, the oxide semiconductor layer 144 is preferably an oxide semiconductor layer which is purified by sufficiently removing an impurity such as hydrogen therefrom or by sufficiently supplying oxygen thereto. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is 5×10¹⁹ atoms/cm³ or less, preferably 5×10¹⁸ atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less, for example. Note that the above hydrogen concentration of the oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). The carrier concentration of the oxide semiconductor layer 144, in which hydrogen is reduced to a sufficiently low concentration so that the oxide semiconductor layer is purified and in which defect states in an energy gap due to oxygen deficiency are reduced by sufficiently supplying oxygen as described above, is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. For example, the off-state current (per unit channel width (1 μm), here) at room temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less. In this manner, by using an i-type (intrinsic) or substantially i-type oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

Note that for the transistor 162 in FIG. 3A, an element isolation region 161 is provided in order to suppress leakage between elements due to miniaturization. Furthermore, the oxide semiconductor layer 144 which is processed in an island shape and smaller than a region surrounded by the element isolation region 161 is used; however, as described in Embodiment 1, a structure in which the oxide semiconductor layer 144 is not processed into an island shape until trenches for element isolation are formed may be employed. When the oxide semiconductor layer is not processed into an island shape, the oxide semiconductor layer can be prevented from being contaminated by etching during processing. It is needless to say that the number of steps can be reduced in the case where the oxide semiconductor layer is not processed into an island shape. In the case of using the oxide semiconductor layer 144 which is processed in an island shape and smaller than the region surrounded by the element isolation region 161, there is no need to divide the oxide semiconductor layer by formation of trenches for element isolation, and thus, the horizontal position of the bottoms of the trenches for element isolation can be shallower than that of the bottoms of trenches for gate electrodes, or the total area of the trenches for element isolation can be reduced.

An insulating layer 151 is provided over the transistor 162, and an electrode 153 which is electrically connected to the gate electrode 148 a is provided over the insulating layer 151. In addition, an insulating layer 152 is provided over the electrode 153. An electrode 154 is provided in an opening formed in the gate insulating layer 146, the insulating layer 150, the insulating layer 151, the insulating layer 152, and the like, and a wiring 156 which is connected to the electrode 154 is formed over the insulating layer 152. Note that although the metal compound region 124, the electrode 142 b, and the wiring 156 are connected to one another through the electrode 126 and the electrode 154 in FIG. 3A, the disclosed invention is not limited thereto. For example, the electrode 142 b may be in direct contact with the metal compound region 124. Alternatively, the wiring 156 may be in direct contact with the electrode 142 b.

Next, an example of a circuit configuration corresponding to FIG. 3A is illustrated in FIG. 3B. Note that in some circuit diagrams, “OS” is written beside a transistor in order to indicate that the transistor includes an oxide semiconductor.

In FIG. 3B, a first wiring (1st Line) is electrically connected to a source electrode of the transistor 160. A second wiring (2nd Line) is electrically connected to a drain electrode of the transistor 160. A third wiring (3rd Line) is electrically connected to one of a source and a drain electrodes of the transistor 162, and a fourth wiring (4th Line) is electrically connected to a gate electrode of the transistor 162. A gate electrode of the transistor 160 and the other of the source and drain electrodes of the transistor 162 are electrically connected to one electrode of a capacitor 164. A fifth wiring (5th Line) is electrically connected to the other electrode of the capacitor 164.

The capacitor 164 can be formed with a pair of electrodes and an insulating layer interposed therebetween and serving as a dielectric, through the same process as the process for manufacturing the transistor 160 and the transistor 162. Note that the present invention is not limited to formation of the capacitor 164 through the same process as the process for manufacturing the transistor 160 and the transistor 162, and layers of the capacitor 164 may be separately provided above the transistor 162. For example, a trench-type capacitor or a stack-type capacitor may be separately formed above the transistor 162 or below the transistor 160 so as to be three-dimensionally stacked, whereby the degree of integration may be increased.

The semiconductor device in FIG. 3B utilizes a characteristic in which the potential of the gate electrode of the transistor 160 can be held, and thus enables data writing, holding, and reading as follows.

Writing and holding of data will be described. First, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third wiring is supplied to the gate electrode of the transistor 160 and to the capacitor 164. That is, predetermined charge is supplied to the gate electrode of the transistor 160 (writing). Here, one of two kinds of charges providing different potentials (hereinafter referred to as a low-level charge and a high-level charge) is applied. After that, the potential of the fourth wiring is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge supplied to the gate electrode of the transistor 160 is held (holding).

In addition, a back gate electrode may be provided, and it is preferable that the transistor 162 be surely a normally off transistor by application of the voltage to the back gate electrode.

This embodiment can be freely combined with Embodiment 1.

Embodiment 3

In this embodiment, a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, which has an unlimited number of write cycles, and which has a structure different from the structure described in Embodiment 2 will be described with reference to FIG. 4.

The semiconductor device illustrated in FIG. 4 includes a transistor 350 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. Although a plurality of transistors is formed using semiconductor materials in the upper and lower portions, the transistor 350 and the transistor 162 will be typically described. Note that FIG. 4 which is taken along line B1-B2 corresponds to a cross-sectional view perpendicular to the channel-length direction of transistors.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics.

The transistor 162 including the second semiconductor material in the upper portion is the same as the transistor 162 described in Embodiments 1 and 2; thus, for description of FIG. 4, the same reference numerals are used for the same parts as those in FIG. 1A.

The transistor 350 formed using the first semiconductor material in the lower portion will be described below.

The transistor 350 includes a semiconductor substrate 310, a gate insulating layer 314, a semiconductor layer 316, a conductive layer 318, a protective insulating layer 320, a sidewall insulating layer 322, impurity regions 324, and an insulating layer 326. Note that the semiconductor layer 316 and the conductive layer 318 function as a gate electrode, and the impurity regions 324 each function as a source region or a drain region.

In addition, the transistor 350 is adjacently provided with shallow trench isolation (STI) regions 312.

The STI regions 312 can be formed as follows: first, trenches (also referred to as grooves) are formed by forming a protective insulating film in a desired region over the semiconductor substrate 310 and performing etching; then, after the formation of the trenches, the trenches are filled with an insulating dielectric film. As the insulating dielectric film, a silicon oxide film, a silicon nitride film, or the like can be used.

Next, the transistor 350 will be described in detail. The gate insulating layer 314 of the transistor 350 can be formed as follows. An insulating film is formed over the semiconductor substrate 310 provided with the STI regions 312, and then, patterning and etching are performed in a desired position, whereby a trench having a depth different from that of the STI regions 312 is formed in the semiconductor substrate 310. After that, heat treatment is performed in an oxygen atmosphere, whereby the semiconductor substrate 310 in the trench is oxidized. In this manner, the gate insulating layer 314 can be formed.

After the gate insulating layer 314 is formed, a silicon film is formed using an LPCVD method or the like. Note that the silicon film is subjected to n⁺ or p⁺ doping treatment, heat treatment, or the like so as to obtain a polysilicon film, whereby a highly conductive semiconductor layer is formed. After that, a metal film is formed over the semiconductor layer by a sputtering method or the like. As the metal film, tungsten, titanium, cobalt, or nickel or an alloy film, a metal nitride film, a silicide film, or the like containing tungsten, titanium, cobalt, or nickel can be used. Patterning is performed on a desired region over the metal film, and etching is performed, whereby the conductive layer 318 is formed. In addition, the semiconductor layer is etched using the conductive layer 318 as a mask, whereby the semiconductor layer 316 can be formed. Note that the conductive layer 318 and the semiconductor layer 316 function as a gate electrode of the transistor 350.

Next, the protective insulating layer 320 is formed over the conductive layer 318. The protective insulating layer 320 can be formed in such a manner that a silicon oxide film, a silicon nitride film, or the like is formed using a plasma CVD method or the like and patterning and etching treatments are performed on a desired region.

Next, a silicon nitride film is formed using a plasma CVD method or the like so as to cover the semiconductor substrate 310 and the protective insulating layer 320 and is etched back, whereby the sidewall insulating layer 322 can be formed.

Next, the impurity regions 324 are formed by performing doping treatment using the protective insulating layer 320 and the sidewall insulating layer 322 as a mask. Note that as a dopant, boron, phosphorus, or the like may be used, and as the impurity regions 324, n⁺ regions, p⁺ regions, or the like can be formed as appropriate depending on the dopant used. Note that the impurity regions 324 each function as a source region or a drain region of the transistor 350.

Next, the insulating layer 326 is formed so as to cover the impurity regions 324, the protective insulating layer 320, and the sidewall insulating layer 322. The insulating layer 326 can be formed using a silicon oxide film or the like by a plasma CVD method or the like.

Next, openings are provided in desired regions of the insulating layer 326, and a connection electrode 325 and a connection electrode 331 are formed so as to be electrically connected to the impurity regions 324. Note that after the connection electrode 325 and the connection electrode 331 are formed, CMP treatment or the like may be performed to planarize surfaces of the insulating layer 326, the connection electrode 325, and the connection electrode 331.

Next, a conductive film is formed using a sputtering method or the like over the insulating layer 326, the connection electrode 325, and the connection electrode 331, and patterning and etching are performed on a desired region, whereby an electrode 328 and an electrode 332 are formed. As a material of the electrode 328 and the electrode 332, tungsten, copper, titanium, or the like can be used as appropriate.

Next, an insulating layer 329 is formed over the insulating layer 326, the electrode 328, and the electrode 332. The insulating layer 329 can be formed using a material and a method similar to those for the insulating layer 326.

Through the above-described process, the semiconductor substrate 310 provided with the transistor 350 formed using a first semiconductor material can be formed.

Here, connections between the transistor 350 including the first semiconductor material in the lower portion and the transistor 162 including the second semiconductor material in the upper portion will be described below.

The transistor 350 is electrically connected to the transistor 162 through the impurity region 324, the connection electrode 325, the electrode 328, and a connection electrode 330. On the other hand, another transistor 350 is electrically connected to the wiring 156 through the impurity region 324, the connection electrode 331, the electrode 332, a connection electrode 334, an electrode 336, and a connection electrode 338.

In addition, the gate electrode of the transistor 350 (i.e., the semiconductor layer 316 and the conductive layer 318) is electrically connected to a source electrode of the transistor 162. Note that the connection between the gate electrode of the transistor 350 and the source electrode of the transistor 162 is not illustrated in FIG. 4, and the connection is established in a three-dimensional direction.

As described above, the plurality of memory cells is formed in the upper portion with the transistors including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. On the other hand, for the peripheral circuit, a semiconductor material other than the oxide semiconductor is used. The semiconductor material other than the oxide semiconductor may be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like and is preferably a single crystal semiconductor. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, the transistor including the material other than the oxide semiconductor can favorably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

This embodiment can be implemented in appropriate combinations with the configurations described in the other embodiments.

Embodiment 4

In this embodiment, a semiconductor device which includes the transistor 162 illustrated in FIGS. 1A to 1C, which can hold stored data even when not powered, which has an unlimited number of write cycles, and which has a structure different from the structures described in Embodiments 2 and 3 will be described with reference to FIGS. 5A and 5B and FIG. 6.

FIG. 5A illustrates an example of a circuit configuration of a semiconductor device, and FIG. 5B is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in FIG. 5A will be described, and then, the semiconductor device illustrated in FIG. 5B will be described.

In the semiconductor device illustrated in FIG. 5A, a bit line BL is electrically connected to a source electrode or a drain electrode of the transistor 162, a word line WL is electrically connected to a gate electrode of the transistor 162, and a source electrode or a drain electrode of the transistor 162 is electrically connected to a first terminal of a capacitor 254.

The transistor 162 including an oxide semiconductor has a characteristic of a significantly small off-state current. For that reason, a potential of the first terminal of the capacitor 254 (or a charge accumulated in the capacitor 254) can be held for an extremely long period by turning off the transistor 162. Further, in the transistor 162 including an oxide semiconductor, a short-channel effect is not likely to be caused, which is advantageous.

Next, writing and holding of data in the semiconductor device (a memory cell 250) illustrated in FIG. 5A will be described.

First, the potential of the word line WL is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor 254 (writing). After that, the potential of the word line WL is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the charge at the first terminal of the capacitor 254 is held (holding).

Because the off-state current of the transistor 162 is extremely small, the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor) can be held for a long time.

Next, reading of data will be described. When the transistor 162 is turned on, the bit line BL which is in a floating state and the capacitor 254 are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor 254. As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor 254 (or the charge accumulated in the capacitor 254).

For example, the potential of the bit line BL after charge redistribution is (C_(B)*V_(B0)+C*V)/(C_(B)+C), where V is the potential of the first terminal of the capacitor 254, C is the capacitance of the capacitor 254, C_(B) is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V_(B0) is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell 250 is in either of two states in which the potentials of the first terminal of the capacitor 254 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case of holding the potential V₁ (=(C_(B)*V_(B0)+C*V₁)/(C_(B)+C)) is higher than the potential of the bit line BL in the case of holding the potential V₀ (=(C_(B)*V_(B0)+C*V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read.

As described above, the semiconductor device illustrated in FIG. 5A can hold charge that is accumulated in the capacitor 254 for a long time because the off-state current of the transistor 162 is extremely small. In other words, refresh operation becomes unnecessary or the frequency of the refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. Moreover, stored data can be held for a long period even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 5B will be described.

The semiconductor device illustrated in FIG. 5B includes a memory cell array 251 including a plurality of memory cells 250 illustrated in FIG. 5A as memory elements in the upper portion, and a peripheral circuit 253 in the lower portion which is necessary for operating the memory cell array 251.

In the structure illustrated in FIG. 5B, the peripheral circuit 253 can be provided under the memory cell array 251. Thus, the size of the semiconductor device can be decreased.

Next, a specific structure of the semiconductor device illustrated in FIG. 5B will be described with reference to FIG. 6.

The semiconductor device illustrated in FIG. 6 includes a memory cell 452 formed in the upper portion, and a peripheral circuit 400 in the lower portion. The peripheral circuit 400 in the lower portion includes a transistor 450 including a first semiconductor material, and the memory cell 452 formed in the upper portion includes a transistor 162 including a second semiconductor material. Note that FIG. 6 which is taken along line C1-C2 corresponds to a cross-sectional view perpendicular to the channel-length direction of transistors.

Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, the first semiconductor material can be a semiconductor material (such as silicon) other than an oxide semiconductor, and the second semiconductor material can be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics.

The transistor 162 including the second semiconductor material in the upper portion is the same as the transistor 162 described above in Embodiments 1 to 3; thus, for description of FIG. 6, the same reference numerals are used for the same parts as those in FIG. 1A and are not described in detail. Here, the transistor 450 including the first semiconductor material in the lower portion will be described below.

The transistor 450 in FIG. 6 includes a channel formation region 404 provided in a substrate 402 including a semiconductor material (such as silicon), impurity regions 406 and high-concentration impurity regions 408 (collectively, simply also referred to as impurity regions) provided such that the channel formation region 404 is sandwiched therebetween, metal compound regions 410 provided in contact with the high-concentration impurity regions 408, a gate insulating layer 411 provided over the channel formation region 404, a gate electrode 412 provided in contact with the gate insulating layer 411, and a source or drain electrode 418 a and a source or drain electrode 418 b electrically connected to the impurity regions.

Here, a sidewall insulating layer 414 is provided on a side surface of the gate electrode 412. Further, an element isolation insulating layer 403 is formed on the substrate 402 so as to surround the transistor 450, and an interlayer insulating layer 420 and an interlayer insulating layer 422 are formed so as to cover the transistor 450. The source or drain electrode 418 a and the source or drain electrode 418 b are electrically connected to the metal compound regions 410 through openings formed in the interlayer insulating layer 420 and the interlayer insulating layer 422. In other words, the source or drain electrode 418 a and the source or drain electrode 418 b are electrically connected to the high-concentration impurity regions 408 and the impurity regions 406 through the metal compound regions 410. Note that in some cases, the sidewall insulating layer 414 is not formed, in order to achieve a higher degree of integration of the transistor 450 or the like. In addition, a connection electrode layer 424 a, a connection electrode layer 424 b, and a connection electrode layer 424 c which are electrically connected to the source or drain electrode 418 a and the source or drain electrode 418 b of the transistor 450 are provided over the interlayer insulating layer 422, and planarization is achieved with an insulating layer 425 which covers the interlayer insulating layer 422, the connection electrode layer 424 a, the connection electrode layer 424 b, and the connection electrode layer 424 c.

The connection electrode layer 424 c is electrically connected to an electrode 428 through a connection electrode 426. Note that the electrode 428 is formed using the same layer as the source electrode and the drain electrode of the transistor 162. In addition, a wiring 156 is electrically connected to an electrode 428 through a connection electrode 430. With the connection electrode layer 424 c, the connection electrode 426, the electrode 428, the connection electrode 430, and the wiring 156, an electrical connection between the peripheral circuit 400 and the memory cell 452, or the like can be established.

In addition, FIG. 6 illustrates, as an example, the semiconductor device in which the memory cell 452 and the peripheral circuit 400 are connected through the connection electrode layer 424 c and the electrode 428; however, the present invention is not limited thereto. Two or more wiring layers and electrodes may be provided between the memory cell 452 and the peripheral circuit 400.

As described above, the memory cell is formed in the upper portion with a transistor including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption. On the other hand, for the peripheral circuit, a semiconductor material other than the oxide semiconductor is used. The semiconductor material other than the oxide semiconductor may be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like and is preferably a single crystal semiconductor. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, the transistor including the material other than the oxide semiconductor can favorably realize a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

This embodiment can be implemented in appropriate combinations with the configurations described in the other embodiments.

Embodiment 5

In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or electronic books will be described with reference to FIGS. 10A and 10B and FIGS. 11 to 13.

In a portable device such as a cellular phone, a smartphone, or an electronic book, an SRAM or a DRAM is used so as to store image data temporarily. The reason why an SRAM or a DRAM is used is that a flash memory is slow in responding and is not suitable for image processing. On the other hand, an SRAM or a DRAM has the following characteristics when used for temporary storage of image data.

In an ordinary SRAM, as illustrated in FIG. 10A, one memory cell includes six transistors, that is, transistors 801 to 806, which are driven with an X decoder 807 and a Y decoder 808. The transistor 803 and the transistor 805, and the transistor 804 and the transistor 806 form inverters, which enables high-speed driving. However, because one memory cell includes six transistors, a large cell area is one disadvantage. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F² to 150 F². Therefore, the price per bit of an SRAM is the most expensive among memory devices.

In a DRAM, as illustrated in FIG. 10B, a memory cell includes a transistor 811 and a storage capacitor 812, which are driven with an X decoder 813 and a Y decoder 814. One cell is configured with one transistor and one capacitor and has a small area. The area of a memory cell in a DRAM is generally 10 F² or less. Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed.

On the other hand, the memory cell of the semiconductor device described in any of the above embodiments has an area of approximately 10 F² and does not need to be refreshed frequently. Therefore, the area of a memory cell can be decreased, and power consumption can be reduced.

Next, FIG. 11 is a block diagram of a portable device. The portable device illustrated in FIG. 11 includes an RF circuit 901, an analog baseband circuit 902, a digital baseband circuit 903, a battery 904, a power supply circuit 905, an application processor 906, a flash memory 910, a display controller 911, a memory circuit 912, a display 913, a touch sensor 919, an audio circuit 917, a keyboard 918, and the like. The display 913 includes a display portion 914, a source driver 915, and a gate driver 916. The application processor 906 includes a CPU 907, a DSP 908, and an interface 909 (IF 909). In general, the memory circuit 912 includes an SRAM or a DRAM. By employing the semiconductor device described in any of the above embodiments for that portion, data can be written and read at high speed and can be held for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 12 illustrates an example of using the semiconductor device described in any of the above embodiments in a memory circuit 950 for a display. The memory circuit 950 illustrated in FIG. 12 includes a memory 952, a memory 953, a switch 954, a switch 955, and a memory controller 951. The memory circuit 950 is connected to a display controller 956 that reads and controls image data input through a signal line (input image data) and data stored in the memory 952 and the memory 953 (stored image data), and is also connected to a display 957 that displays an image based on a signal input from the display controller 956.

First, image data (input image data A) is produced by an application processor (not illustrated). The input image data A is stored in the memory 952 through the switch 954. Then, the image data stored in the memory 952 (stored image data A) is transmitted to the display 957 through the switch 955 and the display controller 956, and is displayed on the display 957.

When the input image data A remains unchanged, the stored image data A is read from the memory 952 through the switch 955 by the display controller 956 normally at a frequency of approximately 30 Hz to 60 Hz.

Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory 953 through the switch 954. Also during that time, the stored image data A is regularly read from the memory 952 through the switch 955. After the completion of storing the new image data (the stored image data B) in the memory 953, from the next frame for the display 957, the stored image data B starts to be read, transmitted to the display 957 through the switch 955 and the display controller 956, and displayed on the display 957. This reading operation continues until the next new image data is stored in the memory 952.

By alternately writing and reading image data to and from the memory 952 and the memory 953 as described above, images are displayed on the display 957. Note that the memory 952 and the memory 953 are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory 952 and the memory 953, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced.

Next, FIG. 13 is a block diagram of an electronic book. FIG. 13 includes a battery 1001, a power supply circuit 1002, a microprocessor 1003, a flash memory 1004, an audio circuit 1005, a keyboard 1006, a memory circuit 1007, a touch panel 1008, a display 1009, and a display controller 1010.

Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit 1007 in FIG. 13. The memory circuit 1007 has a function to temporarily hold the contents of a book. For example, a user may use a highlight function. In some cases, a user wants to mark a specific portion while reading an electronic book. This marking function is called highlight function and is used to make a difference from the other portions by changing the display color, underlining, making characters bold, changing the font of characters, or the like. The function makes it possible to store and hold data of a portion specified by a user. In order to store the data for a long time, the data may be copied to the flash memory 1004. Also in such a case, by employing the semiconductor device described in any of the above embodiments, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced.

As described above, the portable devices described in this embodiment each incorporates the semiconductor device according to any of the above embodiments. Therefore, it is possible to obtain a portable device which is capable of reading data at high speed, holding data for a long time, and reducing power consumption.

The configurations, methods, and the like described in this embodiment can be combined as appropriate with any of the configurations, methods, and the like described in the other embodiments.

Example 1

In this example, calculations were carried out to determine whether or not a short-channel effect is caused in the transistor having a trench structure which is described in Embodiment 1.

For the calculations, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used.

FIG. 7A shows a structure used for the calculation and the sizes of components. The thickness of the gate insulating layer is set to 5 nm, the thickness of the oxide semiconductor layer is set to 5 nm, and the depth of the trench for the gate electrode is set to 0.4 μm. FIG. 7A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 90 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 110 nm A material of the oxide semiconductor layer is an In—Ga—Zn—O-based oxide semiconductor (with a band gap of 3.15 eV, an electron affinity of 4.6 eV, and an electron mobility of 10 cm²/Vs), the work function of the electrodes in contact with the oxide semiconductor layer (the source electrode and the drain electrode) is 4.6 eV, and the work function of the gate electrode is 5.5 eV. FIG. 7B shows the result of a calculation of Vg-Id characteristics of the transistor having the trench structure (with Vds=1 V at a temperature of 27° C.).

FIG. 8A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 60 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 80 nm FIG. 8B shows the result of a calculation carried out with the same conditions as in FIG. 7B except the length of the bottom of the trench and the distance between the source electrode and the drain electrode.

FIG. 9A shows a transistor having a trench structure in which the length of the bottom of the trench (the length in the channel-length direction) is 30 nm and the distance between the source electrode and the drain electrode (the length in the channel-length direction) is 50 nm. FIG. 9B shows the result of a calculation carried out with the same conditions as in FIG. 7B except the length of the bottom of the trench and the distance between the source electrode and the drain electrode.

The results of the calculations show that all the transistors having the structures in FIGS. 7A, 8A, and 9A have substantially the same characteristics. The threshold voltage (Vth) of each transistor is 0.8 V and the subthreshold swing (S value) thereof is 60 mV/dec, which are favorable values.

These calculation results reveal that a short-channel effect such as a negative shift of the threshold voltage or an increase in the subthreshold swing is not caused even when the distance between the source electrode and the drain electrode (the length in the channel-length direction) is decreased to 50 nm, and favorable transistor characteristics are obtained.

For comparison, similar calculations were carried out using transistors having not a trench structure but a planar structure. As the distance between the source electrode and the drain electrode (the length in the channel-length direction) decreased, the channel length also decreased. A short-channel effect such as a negative shift of the threshold voltage or an increase in the subthreshold swing was caused. Furthermore, an increase in leakage current (off-state current) generated when a negative bias was applied to the gate was also observed.

Compared with the results of the comparative calculations, the results of the calculations in FIGS. 7B, 8B, and 9B are favorable. With the transistor structure described in Embodiment 1, the change in substantial channel length is small even when the distance between the source electrode and the drain electrode (the length in the channel-length direction) is decreased. Therefore, a short-channel effect is not caused, and off-state current can be small. Accordingly, a memory cell having favorable retention characteristics can be produced.

EXPLANATION OF REFERENCE

-   -   100: substrate, 108: gate insulating layer, 110: gate electrode,         116: channel formation region, 120: impurity region, 124: metal         compound region, 126: electrode, 130: insulating layer, 142 a,         142 b: electrode, 143 a, 143 b: insulating layer, 144: oxide         semiconductor layer, 146: gate insulating layer, 148 a, 148 b:         gate electrode, 149: insulating layer, 150: insulating layer,         151: insulating layer, 152: insulating layer, 153: electrode,         154: electrode, 156: wiring, 160: transistor, 161: element         isolation region, 162: transistor, 163: transistor, 164:         capacitor, 165: element isolation region, 250: memory cell, 251:         memory cell array, 253: peripheral circuit, 254: capacitor, 310:         semiconductor substrate, 312: STI region, 314: gate insulating         layer, 316: semiconductor layer, 318: conductive layer, 320:         protective insulating layer, 322: sidewall insulating layer,         324: impurity region, 325: connection electrode, 326: insulating         layer, 328: electrode, 329: insulating layer, 330: connection         electrode, 331: connection electrode, 332: electrode, 334:         connection electrode, 336: electrode, 338: connection electrode,         350: transistor, 400: peripheral circuit, 402: substrate, 403:         element isolation insulating layer, 404: channel formation         region, 406: impurity region, 408: high-concentration impurity         region, 410: metal compound region, 411: gate insulating layer,         412: gate electrode layer, 414: sidewall insulating layer, 418         a: source or drain electrode, 418 b: source or drain electrode,         420: interlayer insulating layer, 422: interlayer insulating         layer, 424 a: connection electrode layer, 424 b: connection         electrode layer, 424 c: connection electrode layer, 425:         insulating layer, 426: connection electrode, 428: electrode,         430: connection electrode, 450: transistor, 452: memory cell,         801: transistor, 803: transistor, 804: transistor, 805:         transistor, 806: transistor, 807: X decoder, 808: Y decoder,         811: transistor, 812: storage capacitor, 813: X decoder, 814: Y         decoder, 901: RF circuit, 902: analog baseband circuit, 903:         digital baseband circuit, 904: battery, 905: power supply         circuit, 906: application processor, 907: CPU, 908: DSP, 909:         interface, 910: flash memory, 911: display controller, 912:         memory circuit, 913: display, 914: display portion, 915: source         driver, 916: gate driver, 917: audio circuit, 918: keyboard,         919: touch sensor, 950: memory circuit, 951: memory controller,         952: memory, 953: memory, 954: switch, 955: switch, 956: display         controller, 957: display, 1001: battery, 1002: power supply         circuit, 1003: microprocessor, 1004: flash memory, 1005: audio         circuit, 1006: keyboard, 1007: memory circuit, 1008: touch         panel, 1009: display, and 1010: display controller.

This application is based on Japanese Patent Application serial no. 2011-014627 filed with Japan Patent Office on Jan. 26, 2011, the entire contents of which are hereby incorporated by reference. 

The invention claimed is:
 1. A semiconductor device comprising: a semiconductor substrate; a first trench in the semiconductor substrate; an impurity region in the semiconductor substrate; a first insulating layer in contact with an inner wall surface of the first trench; a first gate electrode in the first trench and over the first insulating layer; a second insulating layer over the first gate electrode and the semiconductor substrate; a third insulating layer over the second insulating layer; an oxide semiconductor layer over and in contact with the third insulating layer; a fourth insulating layer over and in contact with the oxide semiconductor layer; and a second gate electrode adjacent to the oxide semiconductor layer with the fourth insulating layer provided therebetween.
 2. The semiconductor device according to claim 1, wherein the semiconductor substrate comprises silicon.
 3. The semiconductor device according to claim 1, further comprising a second trench in the third insulating layer, wherein the oxide semiconductor layer is provided in the second trench.
 4. The semiconductor device according to claim 1, further comprising a source electrode and a drain electrode in contact with the oxide semiconductor layer, wherein one of the source electrode and the drain electrode is electrically connected to the first gate electrode.
 5. The semiconductor device according to claim 1, further comprising a source electrode and a drain electrode in contact with the oxide semiconductor layer, wherein one of the source electrode and the drain electrode is electrically connected to a conductive layer, and wherein the conductive layer is electrically connected to the impurity region through an opening in the second insulating layer.
 6. The semiconductor device according to claim 1, wherein the oxide semiconductor layer comprises indium, zinc and a metal other than indium and zinc, and wherein the oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer.
 7. A semiconductor device comprising: a first transistor comprising: a semiconductor substrate; a first trench in the semiconductor substrate; a source region and a drain region in the semiconductor substrate; a first insulating layer in contact with an inner wall surface of the first trench; and a first gate electrode in the first trench and over the first insulating layer; a second insulating layer over the first transistor; a third insulating layer over the second insulating layer; and a second transistor comprising: an oxide semiconductor layer over and in contact with the third insulating layer; a source electrode and a drain electrode over and in contact with the oxide semiconductor layer; a fourth insulating layer over the source electrode and the drain electrode and in contact with the oxide semiconductor layer; and a second gate electrode over the oxide semiconductor layer with the fourth insulating layer provided therebetween.
 8. The semiconductor device according to claim 7, wherein the semiconductor substrate comprises silicon.
 9. The semiconductor device according to claim 7, further comprising a second trench in the third insulating layer, wherein the oxide semiconductor layer is provided in the second trench.
 10. The semiconductor device according to claim 7, wherein one of the source electrode and the drain electrode is electrically connected to the first gate electrode.
 11. The semiconductor device according to claim 7, wherein one of the source electrode and the drain electrode is electrically connected to a conductive layer, and wherein the conductive layer is electrically connected to one of the source region and the drain region through an opening in the second insulating layer.
 12. The semiconductor device according to claim 7, wherein the oxide semiconductor layer comprises indium, zinc and a metal other than indium and zinc, and wherein the oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer.
 13. A semiconductor device comprising: a circuit comprising a first transistor comprising: a semiconductor substrate; a first trench in the semiconductor substrate; an impurity region in the semiconductor substrate; a first insulating layer in contact with an inner wall surface of the first trench; and a first gate electrode in the first trench and over the first insulating layer; and a second insulating layer over the first gate electrode and the semiconductor substrate; a memory element comprising a second transistor over and electrically connected to the circuit, the second transistor comprising: a third insulating layer over the second insulating layer; an oxide semiconductor layer over and in contact with the third insulating layer; a source electrode and a drain electrode in contact with the oxide semiconductor layer; a fourth insulating layer over and in contact with the oxide semiconductor layer; and a second gate electrode adjacent to the oxide semiconductor layer with the fourth insulating layer provided therebetween.
 14. The semiconductor device according to claim 13, wherein the semiconductor substrate comprises silicon.
 15. The semiconductor device according to claim 13, further comprising a second trench in the third insulating layer, wherein the oxide semiconductor layer is provided in the second trench.
 16. The semiconductor device according to claim 13, wherein one of the source electrode and the drain electrode is electrically connected to the first gate electrode.
 17. The semiconductor device according to claim 16, further comprising a capacitor, wherein the one of the source electrode and the drain electrode is electrically connected to one electrode of the capacitor.
 18. The semiconductor device according to claim 13, wherein one of the source electrode and the drain electrode is electrically connected to a conductive layer, and wherein the conductive layer is electrically connected to the impurity region through an opening in the second insulating layer.
 19. The semiconductor device according to claim 13, wherein the oxide semiconductor layer comprises indium, zinc and a metal other than indium and zinc, and wherein the oxide semiconductor layer includes a crystal having a c-axis substantially perpendicular to a surface of the oxide semiconductor layer. 